Devices such as cellular phones, digital cameras, PDA, solar batteries, or touch panel displays are widely used, and show a tendency of further prevalence.
Conventionally, for those applications, resin substrates made of acryl and the like are used as a protective member for protecting a display. However, the acrylic resin substrate easily bends, because of its low Young's modulus, when the screen of a display is pushed by a pen, human finger and the like, thus, the resin substrate comes into touch with an internal display to generate poor display, in some cases. There is also a problem in that flaw is easily formed on the surface of the acrylic resin substrate, and visibility tends to deteriorate. One method for solving those problems is to use a glass substrate as a protective member. The glass substrate (cover glass) to be used as the protective member is required (1) to have high mechanical strength, (2) to be low in density and light in weight, (3) to be cheap and to be supplied in a large amount, (4) to have excellent bubble quality, (5) to have high optical transmittance in the visible region, and (6) to have high Young's modulus to prevent bending in pushing the surface with a pen, finger, and the like. In particular, when the requirement (1) is not satisfied, a role as the protective member is not attained, thus, glass substrates tempered by ion exchange and the like (so-called tempered glass substrate) are conventionally used (see Patent Document 1, Non-patent Document 1).
Patent Document 1: JP 2006-83045 A
    Non-patent Document 1: Tetsuro Izumitani et al., “New glass and physicality thereof”, First edition, Management System Laboratory. Co., Ltd., 20 Aug. 1984, p 451-498